We have been studying the phosphorylated neurofilament (PNF) epitope expression in rodent and human brains. PNF immunoreactivity is not generally found in neuronal axons, except in some pathological conditions where abnormal accumulation of neurofilaments induces neurofilament phosphorylation in perikarya of neurons. We have found that cell bodies of neurons of the nucleus tractus mesencephalici nervi trigemini react with antibodies against PNF in rodent brain. We have also examined the hippocampal and brainstem neurons in control brains and those of patients with schizophrenia and Alzheimer's disease. The results suggest the widespread occurrence of perikaryonal PNF immunoreactivity in human brains. It is possible that the hippocampal and brainstem cell body neurons are predilection sites for normal hypophosphorylation.